I know Mother Jones is not well-regarded as a publication without bias, but this article tends to cut past the heart of that. It basically goes on to make the hypothesis that crime rates have fallen over the last few decades as we as a country have cleaned the lead out of our environment (gas, paint, etc.).

So, in the argument of whether Canada is safer or more dangerous because they have more or less guns per capita, there also begs the question of the lead contamination of the country. If Canada has less lead, does that contribute to lower violent crime rates? Vice-versa?